The present invention relates to a control system for a selective ink jet printing element operating through a nozzle of a container provided with a piezoelectric transducer which is capable of compressing or expanding the container when subjected to predetermined voltages. The system is of the kind which comprises an oscillatory circuit which includes the transducer and is normally connected to a dc voltage source and an arhythmic pulse generator for selectively exciting said oscillator circuit.
Control circuits for transducers of selective ink jet printing elements are known, in which a pulse generator is arranged to act on the circuit in such a way as to produce, at the transducer, a variation in voltage such as to expel a droplet. In a known circuit arrangement, the transducer (which appears electrically as a capacitance) is included in a damped oscillatory circuit in which the constant-duration pulse from the generator forms a complex voltage wave, with a rapid rise and a slow fall, which reduces the maximum printing frequency. In addition, such a wave is affected by harmonics associated with the resonance frequency of the control circuit, which give rise to oscillations at the meniscus of the ink in the nozzle, whereby the characteristics of the droplet depend on the moment at which it is discharged.
In another known circuit, the oscillatory circuit of the transducer is a parallel resonant circuit which comprises the secondary winding of a transformer, whereby the transducer is normally completely de-energised. The constant-duration pulse of the generator produces, in the oscillatory circuit, a voltage wave which oscillates about the value zero, followed by damped secondary waves which maintain the meniscus in an agitated condition. In order to allow these waves to die away sufficiently it is necessary in this case also to reduce the maximum frequency of printing.
In addition, the oscillatory circuit being of parallel-type, that circuit will generate a pressure wave with a very high proportion of frequency harmonics higher than its resonance frequency, which will excite the frequency at nodal diameters of the meniscus and give rise to interference in the discharge of the ink drops.